Pseudomonas aeruginosa is an important cause of pneumonia and sepsis in neutropenic children and adults. In cystic fibrosis patients, it causes chronic pulmonary infections leading to respiratory failure and death. P. aeruginosa also infects burns surgical and corneal wounds. Virulence of P. aeruginosa is multifactorial and involves both cell associated structures such as pili and a number of secreted products such as exotoxin A and proteases. Expression of these virulence factors is highly regulated which probably accounts for the ability of P. aeruginosa to cause such a wide variety of infections in vastly different host environments. The molecular mechanisms by which these products are regulated are poorly understood. The broad objective of this investigation is to increase our understanding of how protease production is regulated in P. aeruginosa. Our recent work has shown that expression of three of the four known P. aeruginosa protease genes (lasA, lasB and aprA) encoding the extracellular proteases, LasA, elastase and alkaline protease, are positively regulated, in trans, by the lasR protein. LasR is homologous to the well characterized luxR protein. Recently, we have also cloned and sequenced the lasI gene, adjacent, to lasR, and found its protein to be homologous to Luxl. The existence of lasR and lasI provides P. aeruginosa, a system by which it can communicate to other (P. aeruginosa) cells, to begin to express proteases which, in turn, leads to host tissue damage and bacterial dissemination. This is a novel concept in bacterial pathogenesis. We now wish to: 1. determine if LasR regulates expression of the lasI gene; 2. determine the relative contribution of LasI and LasR to expression of the protease genes; 3. study the structure and function of the LasR protein and; 4. begin to characterize the regions of the lasB, lasA and aprA genes which are influenced by LasR and LasI. The results of these experiments should provide insight into how protease production is regulated in P. aeruginosa and, thereby, enhance our knowledge on virulence regulation in this microorganism. Defining the variables that regulate virulence of P. aeruginosa may lead to the identification of potential new targets for antimicrobial therapy and/or the development of reagents for monitoring the expression of these virulence factors.